Surface studies on uranium monocarbide using XPS and SIMS

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Journal of Nuclear Materials 224 (1995) 25-30

jouroalof nuclear materials

Surface studies on uranium monocarbide using XPS and SIMS R. Asuvathraman a, S. Rajagopalan a K. Ananthasivan a, C.K. Mathews a, R.M. Mallya b a

Chemical Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India b Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India

Received 14 December 1994; accepted 15 February 1995

Abstract The air-exposed surfaces of sintered and arc-melted UC samples were examined by XPS and SIMS. XPS results indicate that the surface is covered with a very thin layer of UO 2 mixed with free carbon, which would have formed along with the oxide during the reaction between UC and oxygen or moisture. From the SIMS depth profile of oxygen, the thickness of the oxide layer is found to be approximately 10 nm. The SIMS oxygen images of the surface as a function of etching time reveal that the surface of UC consists of a top layer of adsorbed moisture/oxygen; this contamination layer is followed by a layer containing uranium oxide, uranium hydroxide and free carbon and then grain boundary oxide and finally bulk UC. The behaviour of sintered and arc-melted samples is similar. I. Introduction The carbides of uranium and plutonium are fuels in fast breeder reactors on account of their excellent thermophysical properties such as high melting point, high thermal conductivity, lack of phase transformation in the temperature range of interest and dimensional stability under irradiation combined with high metal atom density. The drawback, however, is that they react rapidly with oxygen and moisture. The oxidation behaviour of uranium monocarbide (UC) has been studied extensively at high temperatures with the conventional techniques of thermogravimetry, metallography and X-ray diffraction (XRD) analysis. Owing to their inherently macroscopic character, these techniques are not suitable for the determination of very thin oxide layers that would be formed on UC surface when exposed to air at room temperature. Very few reports are available on the study of initial stages of UC oxidation in air at room temperature on the microscopic scale. Camagni et al. [1] employed an optical method based on ellipsometry to measure the thickness of the oxide layer formed on single crystal UC surface when exposed to purified oxygen of 4000 Pa pressure at five different temperatures in the range 333-433 K. The

resolution of the technique was 0.5 nm and the relative uncertainty was < 5%. It was found that the isothermal growth of the oxide layer on single crystal UC was essentially parabolic, with the rate constant depending on temperature at a fixed oxygen pressure. Matzke [2] used the Rutherford backscattering technique to measure the thickness of the surface oxide formed on single crystals of UC when exposed to air with different relative humidity at room temperature. It was reported that at low relative humidity, fast oxidation was found initially followed by a parabolic growth of an oxide layer, suggesting that the oxidation was controlled by oxygen diffusion through the oxide layer. The present study has been taken up to characterize the surfaces of sintered and arc-melted uranium carbide pellets in order to get an understanding of their oxidation behaviour in the laboratory atmosphere at room temperature. Since carbide pellets are stored at room temperature, this study has a direct practical application in assessing the effect of storage. 2. Experimental Sintered UC pellets were prepared by the carbothermic reduction of U 3 0 8 powder. Stoichiometric

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R. Asuvathraman et aL /Journal of Nuclear Materials 224 (1995) 25-30

amounts (1 : 10 mole ratio) of U 3 0 8 and nuclear reactor grade graphite powder were mixed in a planetary ball mill, pelletized and heated at 1273 K for 2 h, in flowing argon gas atmosphere, to reduce U 3 0 8 to U O 2 according to the reaction U 30 s + C ~ 3UO 2 + C O 2. The pellets were then heated at 1873 K for 5 h, in flowing argon gas atmosphere, to form UC according to the equation 3UO 2 + 9C --* 3UC + 6CO. The overall reaction for the preparation of UC from U 3 0 8 is thus U308 + 10C ~ 3UC + 6CO + CO 2. The samples were characterized by X-ray diffraction (XRD) and metallography. The oxygen content in the samples were analysed by using the oxygen analyser supplied by M / s . Leco Corp., USA. Arc-melted samples were obtained by melting the sintered pellets under an inert atmosphere in a water-cooled, copperhearth tri-arc furnace. The pellets were ground on a fine SiC emery to remove any existing surface oxide and polished by using diamond paste and a non-aqueous lubricant. It was then ultrasonically cleaned with isopropyl alcohol. During the surface treatment the samples were handled in air. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) studies were performed on these samples. XR D patterns for the samples were recorded by using a Siemens DS00 powder X-ray diffractometer equipped with a graphite curved crystal secondary beam monochromator and Cu Kc~ radiation. The XPS spectra were recorded by using the VG ESCA LAB MKII instrument. Monochromatic Mg K s radiation was employed. The Mg target X-ray tube was operated at 10 kV acceleration potential and at 10 mA current. The analyzer was operated in the constant analyzer energy mode with a pass energy of 50 eV. The source and collector slits were set to 6 mm. When recording the spectra the main chamber was at a base pressure of 1.3 × 10 -6 Pa. Before recording the spectra, the surface of the sample was cleaned by using an Ar + ion beam operated at 8 keV and 40 izA, at a pressure of 1.3 x 10 -3 Pa. The etching was done repeatedly till there was no further decrease in the O(ls) signal. The spectrometer calibration was checked by recording the Cu(2p) spectra from a standard Cu stud. Sample charging was observed probably due to poor electrical contact between the sample and the sample support and therefore a secondary calibration was performed by recording the C(ls) spectra before etching the sample and the binding energy of this adventitious carbon was taken to be 284.6 eV [3]. The spectral

resolution of the XPS peaks was = 2 eV and the standard error in the peak position was + 0.2 eV. The SIMS spectra were recorded by using a C A M E C A IMS4F instrument. A Cs + ion beam at 10 keV and 5 × 10 - s A was used. The beam was scanned over an area of 250 × 250 ~zm2 and the base pressure of the chamber was 1.3 x 10 -6 Pa.

3. Results and discussion The optical micrographs of the sintered and arcmelted samples revealed the homogeneous single phase nature of the samples. X-ray diffraction patterns of the sintered pellet clearly indicated the formation of single-phase UC. In the case of the arc-melted sample, the presence of a small amount of uranium metal was also observed in the X R D patterns. In both the samples no oxide was detected by XRD. Using the higher angle diffraction lines of U C the lattice parameter was calculated to be 496.3 pm for sintered samples and 496.1 pm for arc-melted samples. The standard error in the lattice parameter measurements is +0.3 pm. The relatively large error may probably be due to the uncertainty in positioning the pellet surface to coincide with the goniometer axis when recording the X R D pattern. From the oxygen analysis, the oxygen content of the sample was found to be < 0.3 mass%. The XPS spectra in the U(4f) region of the sintered and arc-melted samples are given in Fig. 1 Two small

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Fig. 1. U(4f) region XPS spectra for (a) sintered and (b) arc-melted samples.

27

R. Asuuathraman et al. /Journal of Nuclear Materials 224 (1995) 25-30

peaks at around 370 and 397 eV and two intense and broad peaks at 380 and 390 eV were observed in this region. The small peak at 370.5 eV for sintered and arc-melted UC is due to Mg Kot 3 satellite line from the X-ray source. The satellite peak observed at 397.0 eV for the sintered sample and at 397.6 eV for the arcmelted sample which is approximately 7 eV from the U(4fs/2) peak, may be attributed to a shake-up transition from the oxygen-derived 2p band to the U(5f) Fermi level of UO 2 [4]. The peak at 380 eV corresponds to U(4fT/2) and the one at 390 eV corresponds to U(4fs/2). The broadness of these peaks (FWHM = 5 eV) suggests the overlap of two peaks probably due to UO 2 and UC. Since the U(4f7/2) peak, unlike the U(4fs/2) peak, is free from the interference of a satellite peak, an attempt was made to resolve the U(4f7/2) peak into two lines by using a least square fitting program. The line positions of the component peaks are 378.4 and 380.2 eV for the sintered sample and 378.7 and 380.0 eV for the arc-melted samples. The peak at 380.2 eV for the sintered sample and at 380.0 eV for the arc-melted sample match with the binding energy of the U(4fT/2) peak for UO2 reported in the literature [5] and the other peak at 378.4 eV and 378.7 eV observed in the sintered and arc-melted sample, respectively, is assigned to UC. This assignment is consistent with the presence of UO 2 on the pellet surface as indicated by the O(ls) XPS spectra.

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Fig. 3. C(ls) region XPS spectra for (a) sintered and (b) arc-melted samples. The XPS spectra in the O(ls) region for the sintered and arc-melted UC samples is shown in Fig. 2. A single asymmetric peak at 530.6 eV binding energy is observed for both the samples. This peak position corresponds to the O(ls) peak of UO 2 [5]. The asymmetry at 532.4 eV may be due to a hydroxide type of compound present at the surface. The presence of a hydroxide group at the surface was also observed in the SIMS mass spectra. Similar hydroxide species of U was

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28

R. Asuvathraman et al. / Journal of Nuclear Materials 224 (1995) 25-30

also reported by Nornes et al. [6] on uranium metal and by Allen [7] on U O 2. XPS spectra in the C(ls) region of the sintered and arc-melted carbide samples are given in Fig. 3. Two distinct peaks were observed for both the samples. The peak at 283.1 eV of sintered sample and 282.5 eV of arc-melted sample are assigned to the carbon in uranium carbide. The second peak at 285.7 eV corresponds to the free carbon. There is no possibility for the adventitious carbon to be present on the sample surface as the sample was cleaned by sputter etching with Ar + ions for more than half an hour before

recording the spectra. Therefore, the free carbon present on the surface along with U O 2 and U C must have been formed as a result of the reaction between U C and moisture or oxygen or both. Three reactions are possible U C + 0 2 ----)U O 2 + C, U C + 2 H 2 0 ~ U O 2 + C + 2H 2, U C + 2 H 2 0 ~ UO2 + CH4.

(1) (2) (3)

Thermodynamic calculations were performed to examine the feasibility of these reactions at room tempera-

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R. Asuvathraman et al. /Journal of Nuclear Materials 224 (1995) 25-30

ture. The Gibbs energy change at room temperature for reaction (1) was calculated from the Gibbs energy of formation of UO 2 and UC [8] and was found to be -933.5 kJ/mol. The Gibbs energy change for reaction (2) was read from literature [9] to be -392.5 kJ/mol and that of reaction (3) was calculated from the Gibbs energy change for reaction (2) and the Gibbs energy of formation of CH 4 [9] to be -443.3 kJ/mol. All these reactions are possible at room temperature as the Gibbs energy changes are negative. The formation of free carbon during the reaction of UC and pure oxygen below 413 K is also reported in the literature [9]. Similar behaviour leading to the formation of free carbon was reported by Larson et al. [10] in the XPS study of PuOxC r in CO 2 atmospheres. The SIMS depth profile of 160- ion for the samples is shown in Fig. 4. The depth profile is similar for both the samples with a peak at around 30 s of etching (etching rate was 10 nm per minute) and it decreases to constant oxygen level within one minute of etching. The initial increase in the x60- signal is probably due to the removal of the contamination layer on the sample surface. In the case of arc-melted UC samples a slight shoulder is observed in the tail region of the peak and this is a clear indication of grain boundary attack by oxygen/moisture. Fig. 5 shows the oxygen ion images obtained by SIMS as a function of etching time. Fig. 5a is the oxygen ion image of the top surface and Fig. 5b and Fig. 5c are the oxygen ion images taken after 60 and 120 s of etching, respectively. The diameter of the circle corresponds to 250 i~m in all the pictures. It can be inferred from the oxygen ion images that the surface is covered with a uniform oxygen containing layer, which could be adsorbed moisture layer, then a layer of low oxygen content, may be a hydrocarbon contamination layer. Beneath this an oxide layer is seen more predominantly along the grain boundaries to some depth and after that a uniform layer of oxygen corre-

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sponding to bulk UC containing dissolved oxygen. This is schematically given in Fig. 6. The oxygen ion images for the sintered sample was similar to that of the arc-melted sample but the grain boundary attack of oxygen/moisture was less pronounced in this sample. This may probably be due to the presence of uranium metal as a second phase in the arc-melted sample which may be present along the grain boundaries and is also getting oxidized.

4. Conclusion Surface chemistry of sintered and arc-melted samples of uranium carbide was investigated by using XPS, SIMS and XRD. XRD patterns of the samples did not show the presence of any oxide phase. XPS and SIMS analysis showed the presence of an oxide layer on the surface. From the binding energy of O(ls) and U(4f) peaks observed in the XPS spectra, the oxide phase was identified as UO 2 and from the depth profile analysis of oxygen by using SIMS the average thickness of the oxide layer was estimated to be approximately 10 nm. The presence of free carbon in the oxide layer was identified from the binding energy of the two peaks observed in the C(ls) region of the XPS spectra. This free carbon must have formed along with the oxide due to the reaction between UC and moisture or oxygen in air. The SIMS oxygen images of the samples indicated that the surface of uranium carbide consisted of a top layer of adsorbed moisture/oxygen which was a contamination layer. This was followed by a layer containing uranium oxide, uranium hydroxide and free carbon and then grain boundary oxide and finally bulk UC. The behaviour of sintered and arc-melted samples was similar. For many studies on UC that depends on the surface state, e.g., diffusion, adsorption and desorption studies, extreme care must be exercised to avoid contact with air or moisture. UC samples have to be handled and stored under moisture-free inert atmosphere.

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Acknowledgements

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We thank S. Anthonysamy and I. Kaliappan of Chemical Group and G. Raghavan and A.K. Tyagi, MSD, IGCAR, Kaipakkam for their help during the course of this work.

References

Fig. 6. Schematic of the surface region of air-exposed uranium carbide sample.

[1] P. Camagni, A. Manara and E. Landais, Surf. Sci. 10 (1968) 332. [2] Hi. Matzke, J. Appl. Phys. 40 (1969) 3819.

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R. Asuuathraman et aL / Journal of Nuclear Materials 224 (1995) 25-30

[3] D. Briggs and M.P. Seah, eds., Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (Wiley, New York, 1983) p. 438. [4] G.C. Allen, P.M. Tucker and J.W. Tyler, J. Phys. Chem. 86 (1982) 224. [5] G.C. Allen, J.A. Crafts, M.T. Curtis and P.M. Tucker, J. Chem. Soc. Dalton Trans. (1974) 1296. [6] S.B. Nornes and R.G. Meisenheimer, Surf. Sci. 88 (1979) 191.

[7] G.C. Allen, Philos. Mag. B 51 (1985) 465. [8] D. Kubaschewski and C.B. Alcock, eds., Metallurgical Thermodynamics, 5th ed. (Pergamon, London, 1979) p. 378. [9] B.R.T. Frost, J. Nucl. Mater. 10 (1963) 265. [10] D.T. Larson and J.M. Haschke, Inorg. Chem. 20 (1981) 1945.